This invention relates generally to imaging systems using pixilated detectors, and more particularly to pixilated semiconductor detectors in imaging systems.
Imaging devices, such as gamma cameras and computed tomography (CT) imaging systems, are used in the medical field to detect radioactive emission events for example, gamma rays in the range of 140 keV emanating from a subject, such as a patient and to detect transmission x-rays not attenuated by the subject, respectively. An output, typically in the form of an image that graphically illustrates the distribution of the sources of the emissions within the object and/or the distribution of attenuation of the object is formed from these detections. An imaging device may have one or more detectors that detect the number of emissions, and may have one or more detectors to detect x-rays that have passed through the object. Each of the detected emissions and x-rays may be referred to as a “count,” but the detected emissions may also be counted together as a ‘signal current’. The detector also determines the number of counts received at different spatial positions. The imager then uses the position dependent count tallies to determine the distribution of the gamma sources and x-ray attenuators, typically in the form of a graphical image having different colors or shadings that represent the processed count tallies.
A pixilated semiconductor detector, for example, fabricated from cadmium zinc telluride (CZT), may provide an economical method of detecting the gamma rays and x-rays. Specifically, at least one known imaging system includes a Room Temperature Semiconductor Radiation Detector (RTD) that is utilized to produce an image having a higher image quality. During operation, the RTD converts radiation photons to an electric charge (Q) using at least one of the photoelectric effect, the Compton effect, and/or electron-electron scattering. Converting photons directly to an electric charge facilitates eliminating the steps of light production and light detection and their corresponding inefficiencies that occur in the known scintillator technology. However, to operate at room temperature, RTD's must have sufficiently large Band Gap Energies (BG) to decrease the quantity of free charge carriers (N) in the material and allow the application of a higher bias voltage (Bias High Voltage HV). This allows the detection of signal pulses without producing a background electric current, referred to herein as a dark current (Id). During operation, the dark current can saturate the readout electronics, and/or reduce the signal to noise ratio (SNR) when measuring the signal electric charge (Q). To measure the signal electric charge (Q), detection electrodes and electronics are applied to surfaces of the RTD. Provided the charge mobility (μ) and carrier recombination lifetime (τ) are high enough, the bias high voltage causes the detection of the electric charge (Q) on the electrodes and electronics.
However, known detectors that are fabricated using a Cadmium Zinc Telluride (CZT) material may have a dark current (Id) that is not sufficiently controlled by the larger band gap. Accordingly, at least some known imaging systems include a cooling system to facilitate reducing the free charge carriers (N) and/or reducing the dark current (Id). For example, at least one known imaging system includes a cooling system that utilizes liquid nitrogen to facilitate reducing the free charge carriers (N) and/or reducing the dark current (Id). However, using a liquid nitrogen system is generally impractical for use in a commercial imaging system. Another known system uses circulating chilled water to control CZT and electronics temperatures, but this is also a significant cost in terms of engineering and safety. Moreover, at least one known imaging system utilizes a Peltier element to facilitate reducing the free charge carriers (N) and/or reducing the dark current (Id) which facilitates avoiding the adverse increase of the dark current (Id) that may be generated due to the heat of nearby objects, such as electronics, for example.
Accordingly, while known cooling systems may have a positive effect on reducing the dark current (Id), the cooling systems may have an adverse effect on the charge mobility (μ) and carrier recombination lifetime (τ). For example, when the quantity of impurities and band edge states within the intrinsic semiconductor device increases, i.e. reduced grade detectors and/or doped semiconductor devices, cooling may decrease the charge mobility (μ) and carrier recombination lifetime (τ) by increasing the interaction of the electric charge (Q) with these localized states, referred to as shallow and deep traps. More specifically, when the semiconductor device is fabricated using a Cadmium Zinc Telluride (CZT) materiel, where the charge mobility (μ) and carrier recombination lifetime (τ) product is marginal, such traps may be a limiting factor.